Chimpanzee

Chimpanzee, (Pan troglodytes), species of ape that, along with the bonobo, is most closely related to humans. Chimpanzees inhabit tropical forests and savannas of equatorial Africa from The Gambia in the west to Lake Albert, Lake Victoria, and northwestern Tanzania in the east. Individuals vary considerably in size and appearance, but chimpanzees stand approximately 1–1.7 metres (3–5.5 feet) tall when erect and weigh about 32–60 kg (70–130 pounds). Males tend to be larger and more robust than females. Chimpanzees are covered by a coat of brown or black hair, but their faces are bare except for a short white beard. Skin colour is generally white except for the face, hands, and feet, which are black. The faces of younger animals may be pinkish or whitish. Among older males and females, the forehead often becomes bald and the back becomes gray.

Natural history

Chimpanzees awaken at dawn, and their day is spent both in the trees and on the ground. After a lengthy midday rest, late afternoon is usually the most intensive feeding period. In the trees, where most feeding takes place, chimps use their hands and feet to move about. They also leap and swing by their arms (brachiate) skillfully from branch to branch. Movement over any significant distance usually takes place on the ground. Though able to walk upright, chimpanzees more often move about on all fours, leaning forward on the knuckles of their hands (knuckle walking). At night they usually sleep in the trees in nests they build of branches and leaves. Chimpanzees are unable to swim, but they will wade in water. The chimpanzee diet is primarily vegetarian and consists of more than 300 different items, mostly fruits, berries, leaves, blossoms, and seeds but also birdeggs and chicks, many insects, and occasionally carrion. Chimpanzees also hunt, both alone and in groups, stalking and killing various mammals such as monkeys, duikers, bushbucks, and wild pigs. They also appear to use certain plants medicinally to cure diseases and expel intestinal parasites.

The female chimpanzee bears a single young at any time of year after a gestation period of about eight months. The newborn weighs about 1.8 kg (about 4 pounds), is almost helpless, and clings to the fur of the mother’s belly as she moves. From about 6 months to 2 years, the youngster rides on the mother’s back. Weaning takes place at about 5 years. Males are considered adults at 16 years of age, and females usually begin to reproduce at about 13 years, but often only two offspring survive during her lifetime. The longevity of chimps is about 45 years in the wild and 58 in captivity; however, older individuals have been documented. For example, Cheetah the chimpanzee, an animal actor from the Tarzanmovies of the 1930s and ’40s, was reported to have lived approximately 80 years.

Social behaviour

Chimpanzees are lively animals with more extraverted dispositions than either gorillas or orangutans. They are highly social and live in loose and flexible groups known as communities, or unit groups, that are based on associations between adult males within a home range, or territory. Home ranges of forest-dwelling communities can be as small as a few square kilometres, but home ranges covering hundreds of square kilometres are known among savanna communities. A community can number from 20 or fewer to well over 100 members. Each consists of several subgroups of varying size and unstable composition. Social dominance exists, with adult males being dominant over adult females and adolescent males. Within a community, there are twice or three times as many adult females as adult males; the number of adults is about equal to the number of immature individuals. Communities usually divide into subgroups called parties, which vary widely in size. The dominance hierarchy among male chimpanzees is very fluid; individuals associate with each other and join and leave different subgroups with complete freedom. The dominant (alpha) male of a group can monopolize ovulating females through possessive behaviour. On the other hand, gang attack by subordinate males can expel an alpha male. Males spend all of their lives in the community they are born in, but occasionally a juvenile male may transfer to another community with his mother. In contrast to males, most females leave their group of birth to join a neighbouring group when they mature at around age 11. Female chimpanzees spend most of their time with their young or with other females. Those with dependent offspring are more likely to range alone or in small parties within narrow “core areas.” Females have been known to form coalitions against a bullying adult male or newly immigrated female.

Relations between different chimp communities tend to be hostile. Intruders on a group’s home range may be attacked, and adult males engage in boundary patrol. On rare occasions, a group may invade a neighbouring territory that is much smaller in size, and fatalities among the smaller group result. Infanticide and cannibalism by adult males, and to a lesser extent by adult females, have been observed. Victimized infants are not only those of neighbouring groups but also those born to newly immigrated females. Between- and within-group competition among individuals of the same sex is the likely cause of such violence. Sometimes a male and female will form a consortship, engaging in exclusive mating relationships by leaving other members of the group and staying in the periphery of the group range. This strategy, however, brings increased risk of attack by neighbouring groups.

Chimpanzees exhibit complex social strategies such as cooperation in combat and the cultivation of coalitions and alliances via ranging together, reciprocal grooming, and the sharing of meat (sometimes in exchange for mating opportunities). An alpha male, for instance, may interfere with his rival in grooming with a third party because such a coalition might jeopardize the alpha’s status. On the other hand, the third party might show strategic opportunism in such a situation, since his assistance to either side could determine which of his superiors prevails. Chimpanzees, therefore, appear to have some concept of “trade.” They console, reconcile, and retaliate during fighting and so share emotions and aspects of psychology similar to those found in humans: self-recognition, curiosity, sympathy, grief, and attribution. Although chimps take care of orphaned infants, they also tease handicapped individuals, conceal information that would bring disadvantage to themselves, and manipulate others for their own advantage by expressing deceptive postures, gestures, and facial expressions.

Chimpanzees are highly intelligent and are able to solve many kinds of problems posed to them by human trainers and experimenters. A number of researchers have taught chimpanzees to use sign language or languages based on the display of tokens or pictorial symbols. The implications of these language studies have been contested, however. Critics charge that apes have not acquired true language in the sense of understanding “words” as abstract symbols that can be combined in meaningful new ways. Other investigators maintain that more recent language training has resulted in the chimpanzees’ acquiring a true recognition of “words” as abstractions that can be applied in novel contexts.

Communication between chimps in the wild takes the form of facial expressions, gestures, and a large array of vocalizations, including screams, hoots, grunts, and roars. Males display excitement by standing erect, stamping or swaying, and letting out a chorus of screams. Chimps use louder calls and gestures for long-distance communication (such as drumming on tree buttresses) and quieter calls and facial expressions for short-distance communication. Similarities to human laughter and smiling might be seen in their “play panting” and grinning, respectively.

Various tools are used in several contexts. Chimpanzees “fish” for termites and ants with probes made of grass stalks, vines, branches, peeled bark, and midribs of leaves. They crack hard nuts open by using stones, roots, and wood as hammers or anvils, and they use “leafy sponges” (a handful of folded leaves or moss) to drink water. Branches and leaves are detached and displayed during courtship. In threat displays, chimps throw rocks and drag and throw branches. Sticks are used to inspect dead pythons or other unfamiliar objects that might be dangerous. Leaves are used hygienically in wiping the mouth or other soiled body parts. Chimpanzees also use different tools in succession as a “tool set.” For example, chimpanzees of the Congo basin first dig into termite mounds with a stout stick and then fish for individual termites with a long, slender wand. Tools are also used in combination as “tool composites.” Chimpanzees in the Guinea region push leafy sponges into hollows of trees containing water and then withdraw the wet sponges by using sticks. Chimps thus differ locally in their repertoire of tool use, with younger animals acquiring tool-using behaviours from their elders. Such cultural differences are also seen in food items consumed and in gestural communication. Chimpanzees indeed possess culture when it is defined as the transmission of information from generation to generation via social learning shared by most members of a single age or sex class in a given group.

Chimpanzees’ intelligence, responsiveness, and exuberance have made them ideal nonhuman subjects for psychological, medical, and biological experiments. Young chimpanzees can become very attached to their human trainers, and their expressions of feeling resemble those of humans more closely than any other animal.

chimpanzee social interactionChimpanzees possess a unique culture, and their intelligence, responsiveness, and exuberance have made them ideal nonhuman subjects for study. Encyclopædia Britannica, Inc.

Taxonomy

Genetic analysis suggests that humans and chimps diverged four million to eight million years ago and that at least 98 percent of the human and chimpanzee genomes are identical. Chimpanzees are classified taxonomically as a single species, Pan troglodytes. (The so-called pygmy chimpanzee, or bonobo, is a distinct and separate species, P. paniscus.) Four subspecies of P. troglodytes are recognized: the tschego, or Central African chimpanzee (P. troglodytes troglodytes), also known as the common chimpanzee in continental Europe; the West African, or masked, chimpanzee (P. troglodytes verus), known as the common chimpanzee in Great Britain; the East African, or long-haired, chimpanzee (P. troglodytes schweinfurthii); and the Nigerian-Cameroon chimpanzee (P. troglodytes ellioti, which was formerly classified as P. troglodytes vellerosus).

human evolutionThe divergence of humans and great apes from a common ancestor.Encyclopædia Britannica, Inc.